WARNING: Host distribution "Ubuntu-18.04" has not been validated with this version of the build system; you may possibly experience unexpected failures. It is recommended that you use a tested distribution.Loading cache: 100% |#####################################################| ETA: 00:00:00Loaded 2792 entries from dependency cache.NOTE: Resolving any missing task queue dependencies

Your safest bet is using Ubuntu 16.04, however, I understand that you wish to use Ubuntu 18.04. In that case I would recommend checking that all the required packages listed on the i.MX Yocto User’s Guide are installed in your system. (It could be that some packages are missing or not available in Ubuntu 18.04 and in this case you would need to manually install them if possible).